Behind the surprise exchange of Army Sgt. Bowe Bergdahl for five Taliban detainees were secret negotiations that began three years ago and ended over the weekend with the retrieval of America's last known prisoner of war, rekindling debate over the propriety of such deals with American adversaries.

Happiness at Sgt. Bergdahl's release was overtaken by complaints from U.S. lawmakers about how the deal was struck without Congress's...